Since the discovery of superconducting materials having critical temperatures that exceed the temperature of liquid nitrogen, there has been a concerted effort to utilize these materials for various applications, such as in wires and electronic devices. In order to be commercially viable, these applications require high temperature superconducting materials with a high critical current density. Critical current density, J.sub.c, is the maximum current density a superconductor can carry at a given temperature and magnetic field. One such high temperature superconducting (HTS) material is a composite oxide of RE, Ba and Cu, (ReBCO) and in particular, REBa.sub.2 Cu.sub.3 O.sub.x (wherein RE represents at least one of the following rare earth elements: Y, La, Sm, Nd, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu).
Current materials research aimed at fabricating high temperature superconducting ceramics in conductor configurations for bulk, practical applications, is largely focused on powder-in-tube methods. Such methods have proven quite successful for the Bi--(Pb)--Sr--Ca--Cu--O (BSCCO) family of superconductors due to their unique mica-like mechanical deformation characteristics. In high magnetic fields, however, this family of superconductors is generally limited to applications below 30.degree. K. In the ReBCO TI--(Pb, Bi)--Sr--(Ba)--Ca--Cu--O and Hg--(Pb)--Sr--(Ba)--Ca--Cu--O families of superconductors, some of the compounds have much higher intrinsic limits and can be used at higher temperatures.
It has been demonstrated that these superconductors possess a high J.sub.c at high temperatures when fabricated as single crystals or in essentially single-crystal form as epitaxial films on single crystal substrates such as SrTiO.sub.3 and LaAIO.sub.3. An epitaxial film is one whose crystalline lattice is nearly perfectly aligned with the lattice of the substrate on which it is deposited. These superconductors have so far been intractable to conventional ceramics and materials processing techniques to form long lengths of a polycrystalline conductor with a J.sub.c comparable to epitaxial films. This is primarily because the poor electrical connections at the boundaries between crystalline grains, which is known in the art as the "weak-link" effect.
Thin-film materials having perovskite-like structures are important in superconductivity, ferroelectrics, and electro-optics. Many applications using these materials require, or would be significantly improved by single crystal, c-axis oriented perovskite-like films grown on single-crystal or highly aligned metal or metal-coated substrates. For instance, Y--Ba.sub.2 --Cu.sub.3 --O_(YBCO) is an important superconducting material for the development of superconducting current leads, transmission lines, motor and magnetic windings, and other electrical conductor applications. When cooled below their transition temperature, T.sub.c, superconducting materials have no electrical resistance and carry electrical current without energy dissipation.
One technique for fabricating a superconducting wire or tape is to deposit a YBCO film on a metallic substrate. Superconducting YBCO has been deposited on polycrystalline metals in which the YBCO is c-axis oriented, but not aligned in-plane. To carry high electrical currents, however, the YBCO films must be biaxially textured, preferably c-axis oriented, with effectively no large-angle grain boundaries, since such grain boundaries are detrimental to the current-carrying capability of the material. YBCO films deposited on polycrystalline metal substrates do not generally meet this criterion.
Many electronic, magnetic, or superconductor device applications require control of the grain boundary character of the device materials. For example, grain boundary character is very important in high temperature superconductors. It is known that the critical current density through a grain boundary may be reduced significantly for misorientation angles greater than 5.degree.-10.degree.. It is thus desirable to obtain superconducting deposits in which the number of grain boundaries with misorientation angles greater than 5.degree.-10.degree. is minimized. For conductors in which the superconducting deposit is epitaxial with an underlying metallic or oxide buffer layer or substrate, it is desirable to minimize the number of grain boundaries with misorientations greater than 5.degree.-10.degree.. This is accomplished if the texture of the substrate has grain orientations which vary by no more than 5.degree.-10.degree.. Useful superconducting layers may be obtained using substrates with a larger spread in grain orientation. However, the properties of the superconductor deposit are expected to improve with a biaxially textured substrate having a narrow spread in grain orientation.
The effects of grain boundary characteristics on current transmission have been clearly demonstrated for certain materials, for example, the material known as YBCO. See Dimos, et al. (1988) Phys. Rev. Left. 61:219; and Dimos, et al. (1990) Phys. Rev. Left. 41:4038. For clean, stoichiometric boundaries, the grain boundary critical current (J.sub.c (gb)) appears to be determined primarily by grain boundary misorientation. The dependence of J.sub.c (gb) on misorientation angles for YBCO has been determined by Dimos et al., supra, for grain boundary types which can be formed in epitaxial films on bicrystal substrates. These include [001] tilt, [100] tilt, and [100] twist boundaries. In each case, however, high angle boundaries were found to be weak-linked.
Recently, the Dimos work has been extended to artificially fabricated [001] tilt bicrystals in Tl.sub.2 Ba.sub.2 CaCu.sub.2 O.sub.8 (A. H. Cardona, et al., Appl. Phys. Lett., 62 (4), 411, 1993)), Ndl.sub.0.85 Ce.sub.0.15 CuO.sub.4, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x- (M. Kawasaki, et al., Appl. Phys. Lett., 62 (4), 417 (1993)), and TlBa.sub.2 Ca.sub.2 Cu.sub.2 O.sub.x -(T. Nabatame, et al., Appl. Phys. Lett. 65 (6), 776 (1994)). In each of these cases, it was found that, as in the case of YBCO, J.sub.c depends strongly on grain boundary misorientation angle. Although no measurements have been made on the material known as Bi-2223, data on current transmission across artificially fabricated grain boundaries in the material termed Bi-2212 indicate that most large angle [001] tilt (M. Kawasaki, et al., Appl. Phys. Left., 62 (4), 417 (1993)) and twist boundaries are weak links, with the exception of some coincident site lattice (CSL) related boundaries (N. Tomita, et al., Jpn. J. Appl. Phys., 29 (1990) L30; N. Tomita, et al., Jpn. J. Appl. Phys., 31, L942 (1992); J. L. Wang, et al., Physica C, 230, 189 (1994)). It is likely that the variation in J.sub.c with [001] tilt grain boundary misorientation in materials Bi-2212 and Bi-2223 is similar to that observed in the well characterized cases of YBCO and TI-based superconductors.
Hence, in order to fabricate high temperature superconductors with a high J.sub.c it is necessary to have a good biaxial texture. This has been shown to result in significant improvement in the superconducting properties of YBCO films. Y. Iijima, et al., Appl. Phys., 74, 1905 (1993); R. P. Reade et. al., Appl. Phys. Lett., 61, 2231 (1992); X. D. Wu, et al., Appl. Phys. Lett., 65, 1961 (1994).
Methods have been developed to biaxially texture ReBCO to obtain a high J.sub.c. High J.sub.c 's have been reported in polycrystalline ReBCO in thin films deposited in special substrates on which a biaxially textured non-superconducting oxide buffer layer is first deposited using ion-beam assisted deposition (IBAD) techniques. High J.sub.c 's have also been reported in polycrystalline ReBCO melt-processed bulk material which contains primarily small angle grain boundaries.
Recent developments in biaxially textured metallic substrates such as Rolling Assisted Biaxially Textured Substrates (RABiTS), such as described in U.S. Pat. No. 5,739,086, which is fully incorporated by reference herein, and IBAD on Hastelloy have enabled the fabrication of high J.sub.c high temperature superconductors (HTS) including YBa.sub.2 Cu.sub.3 O.sub.x and (TI, Bi)-1223.
Specifically, in the area of high-temperature superconductor (HTS) wires for power applications, the superconducting materials must be biaxially oriented to assure large critical current densities. These requirements can be satisfied by deposition of HTS films on biaxially oriented substrates. An attractive candidate substrate is thermo-mechanically biaxially textured Ni, which is inexpensive, possesses good mechanical properties, has a high melting temperature (1455.degree. C.), and a relatively good oxidation resistance. Unfortunately, direct epitaxy of HTS materials on Ni substrates is exacerbated by chemical reaction and oxidation of the Ni at high temperatures, and by the large crystalline lattice mismatch with all HTS materials. The present invention solves these problems, providing biaxially oriented buffer layers that are chemically and structurally compatible with the subsequent HTS materials.
In general, biaxially-oriented oxide films are epitaxially grown at high temperature and in an oxygen atmosphere. Such deposition conditions may cause oxidation of a non-oxide substrate surface. These effects degrade texturing of films deposited on metallic substrates. X. D. Wu et al have successfully deposited yttria-stabilized-zirconia (YSZ) buffer layers on polycrystalline Ni substrates at room temperature by using ion beam assisted deposition (IBAD), which can solve the oxidation problem. X. D. Wu et al., "High Current YBa.sub.2 Cu.sub.3 O.sub.7-.delta. Thick Films On Flexible Ni Substrates With Textured Buffer Layers", Appl. Phys. Lett. 65 (15), Oct. 10, 1994, p 1961. However, IBAD is a complex process, which limits its practical applications. In addition, there is no epitaxial relationship between the Ni substrate and the film.
Buffer layers play an important role in superconducting devices, such as a high T.sub.c superconducting laminate tape which consists of high T.sub.c superconducting film, buffer layers, and textured metal substrate. The buffer layers are used not only to transfer the biaxially oriented structure from the substrate to the top high T.sub.c superconducting film, but also to protect the high T.sub.c superconducting film from contamination from the underlying substrate. Insulating buffer layers such as CeO.sub.2 and SrTiO.sub.3 crack during high temperature annealing, reducing the efficiency of the HTS material. Moreover, insulating buffer layer such as CeO.sub.2, SrTiO.sub.3, and YSZ, cannot electrically stabilize the HTS conductor layer during transient loss of superconductivity.
Transient loss of superconductivity may cause the superconducting device to overheat. Unstable transient loss of superconductivity may cause portions of the superconducting material to vaporize. An electrically conducting oxide buffer layer would solve the above problems. However, successful deposition of an electrical conductive oxide buffer layer on a biaxially textured polycrystalline metallic substrate has not been disclosed heretofore.
One particular conducting material, LaNiO.sub.3, is reported by Kumar et al., "LaNiO.sub.3 : A Promising Material for Contact with YBa.sub.2 CU.sub.3 O.sub.7-x Thin Films" IEEE Transactions on Applied Superconductivity, Vol. 5, No. 4, December 1995, p. 3498, to have been successfully deposited on YBCO for use as an electrical contact material. This use of LaNiO.sub.3 as an electrical contact material is a significantly different use than described herein because the deposition of YBCO on the buffer layer must occur under conditions where the YBCO will form (high temperature and oxygen partial pressure). For electrical contacts, the LaNiO.sub.3 can be deposited at lower temperatures on YBCO that has already been formed.
Epitaxial deposition of LaNiO.sub.3 and deposition of oxides on LaNiO.sub.3 has been disclosed in H. Ichinose et al., "Synthesis of PbTiO.sub.3 film on LaNiO.sub.3 -coated substrate by the spray-ICP technique" J. Materials Science 29 (1994) p5115-5120; Aidong Li et al., "Preparation of Epitaxial Metallic LaNiO.sub.3 Films on SrTiO.sub.3 by Metallorganic Decomposition for the Oriented Growth of PbTiO3" Appl. Phys. Lett. 69 (2), Jul. 8, 1996; and K. M. Satyalakshrni et al.,"Epitaxial Metallic LaNiO3 Thin Films Grown by Pulsed Laser Deposition", Appl. Phys. Lett. 62 (11), Mar. 15, 1993, p1233. However, in all of these applications, the epitaxial films are deposited on insulating single crystals.
For further background information, refer to the following publications:
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